1. Field of the Invention
The present invention relates to a microlens array in which a plurality of microlenses are arranged, a method of forming the microlens array, a solid-state image pickup device having the microlens array, and a method of manufacturing the solid-state image pickup device.
2. Description of the Related Art
To realize, for example, a solid-state image pickup device which forms fine images and is small in size and light in weight, it is necessary to decrease the area of each pixel. As a consequence, however, the incident light quantity per pixel decreases, and this lowers the sensitivity. Therefore, a microlens array having microlenses in a one-to-one correspondence with a plurality of photosensitive portions is provided in an on-chip state. Each microlens focuses, toward a photosensitive portion, even light incident on a portion except the photosensitive portion.
FIGS. 1A to 2 show the first related art of a CCD solid-state image pickup device having this microlens array and a method of manufacturing this CCD solid-state image pickup device. In the manufacturing method of this first related art, as shown in FIG. 1A, photosensitive portions 12 are formed in a semiconductor substrate 11, and conductive films 13 used to isolate pixels and as charge transfer electrodes are formed on the semiconductor substrate 11. The conductive films 13 and the like are covered with light-shielding films 14, and the light-shielding films 14 are covered with a planarizing film 15. In addition, a color filter 16 is formed on the planarizing film 15.
Thereafter, a material layer 17 of a microlens array is formed on the color filter 16 by using, e.g., a polystyrene resin or a polyimide resin. The surface of the material layer 17 is coated with a resist 21, and the resist 21 is processed into planar patterns of the microlens array. As shown in FIG. 1B, the resist 21 is formed into a three-dimensional shape of the microlens array by reflow.
In FIG. 1C, the resist 21 and the material layer 17 are simultaneously etched to transfer the three-dimensional shape of the resist 21 to the material layer 17. Consequently, as shown in FIG. 2, a microlens array 23 having microlenses 22 in a one-to-one correspondence with the photosensitive portions 12 is formed.
Note that when the resist 21 and the material layer 17 are simultaneously etched, the material layer 17 is formerly side-etched to produce a negative critical dimension or CD loss by which the planar patterns transferred from the resist 21 to the material layer 17 are smaller than the planar patterns of the resist 21. Recently, however, the planar patterns of the resist 21 are accurately transferred to the material layer 17.
When the resist 21 is processed in the step shown in FIG. 1A, a spacing x of about 0.4 m is formed between the planar patterns of the resist 21 due to the limit of the resolution of lithography. This spacing x can be narrowed by reflow in the step shown in FIG. 1B. However, if the planar patterns of the resist 21 are made too close to each other, the planar patterns of the resist 21 can contact each other due to, e.g., a variation in reflow.
If the planar patterns of the resist 21 contact each other, the resist 21 is planarized by, e.g., the surface tension of the resist 21, and this makes the formation of the microlens array 23 impossible. Therefore, even after reflow, the spacing between the planar patterns of the resist 21 can be narrowed to at most about 0.3 .mu.m.
FIGS. 3A and 3B show vertical and horizontal sections of the second related art of a solid-state image pickup device having a microlens array. In this second related art, photosensitive portions 32 are formed in a semiconductor substrate 31, and conductive films 33 to be used to isolate pixels and as charge transfer electrodes are formed on the semiconductor substrate 31. The conductive films 33 and the like are covered with light-shielding films 34, and the light-shielding films 34 and the like are covered with a planarizing film 35.
A color filer 36 is formed on the planarizing film 35, and microlenses 37 are formed on this color filter 36. The photosensitive portions 32 are arranged in a matrix manner, and the microlenses 37 are formed in a one-to-one correspondence with these photosensitive portions 32. As shown in FIG. 4, the microlenses 37 are also arranged in a matrix manner to form a microlens array 38.
In the first related art shown in FIGS. 1A to 2, the planar patterns of the resist 21 are accurately transferred to the material layer 17. Therefore, as shown in FIG. 2, the spacing between the microlenses 22 is also as wide as about 0.3 .mu.m, so only the microlens array 23 having a large non-focusing region can be formed.
In the microlens array 23 having this large non-focusing region, however, as can be seen from FIG. 2, incident light 24 is not effectively focused on the photosensitive portions 12. Additionally, a large quantity of the incident light 24 is obliquely incident on the photosensitive portions 12 by being reflected by the light-shielding films 14 or the like and enters into portions below the charge transfer electrodes. Accordingly, in this first related art it is difficult to accomplish a solid-state image pickup device with a high sensitivity and little smear.
The pattern of each pixel of a solid-state image pickup device is generally a rectangle rather than a square. Therefore, as shown in FIG. 4, the planar shape of the microlens 37 is also an ellipse or a shape close to an ellipse, rather than a true circle. Nevertheless, in the second related art described above, the microlens 37 has equal heights y in both the vertical and horizontal directions regardless of whether the diameter of the microlens 37 is small as shown in FIG. 3A or large as shown in FIG. 3B.
Accordingly, the curvature of the microlens 37 in the vertical direction differs from that in the horizontal direction. When the focal point of incident light 39 is positioned in the photosensitive portion 32 in the horizontal direction, the focal point of the incident light 39 is not positioned in the photosensitive portion 32 in the vertical direction. Consequently, in the above second related art, the sensitivity is not necessarily sufficiently high although the microlenses 37 are used, and it is difficult to reduce smear or the like. This phenomenon becomes typical as the aspect ratio of the pattern of each pixel increases.
Note that it is being attempted to adjust curvatures by making lens heights in the vertical and horizontal directions substantially different from each other by forming a cylindrical lens continuously extending in the vertical or horizontal direction and further forming separate lenses across this cylindrical lens in a one-to-one correspondence with pixels.
Unfortunately, it is presently very difficult to form two types of lenses overlapped as above. Also, when such two types of lenses are formed, the number of manufacturing steps increases to lower the productivity, and this increases the manufacturing cost of a solid-state image pickup device. Additionally, even when such two types of lenses are used, interfaces exist inside the lenses so incident light is reflected more. Accordingly, it is still difficult to increase the sensitivity and reduce smear or the like.